Save as mesh: one dialog for STL and 3MF, with Send to slicer (#1272)

One docked Save as mesh dialog replaces Export STL / 3MF / 3MF with face tags, with an optional Send to slicer that lists the slicers installed on this machine. Engine: file.save_mesh and file.slicers; the Convert Mesh dialog goes (mesh.convert and the MeshConvert feature stay, so existing designs open). Strings in all catalogs; screenshots in the PR. Checked on main + this branch: cargo check --workspace, the engine slicer/mesh/file tests, the UI catalog and menu tests and scenario s14 (the one local colour-token failure is epaint's TexturesDelta panic, which main shows on this machine too).

crates/engine/src/lib.rs

@@ -19,6 +19,8 @@ pub mod product_scene;
 pub mod recovery;
 pub mod render_jobs;
 pub mod sample;
+mod slicer_icon;
+pub mod slicers;
 pub mod view;
 
 use std::sync::Arc;